I've gone down this road. A heightened magnet definitely impacts gameplay and things are 100% better once it's addressed.
The magnet assembly drops out quite easily -- it just requires unplugging a molex and unscrewing two screws. Depending on how mushroomed the head is, you've got to be very careful about damaging the playfield when you pull the magnet out of the playfield. If it's too big to fit, you've got a bigger problem. (More on that in a minute.)
Once the magnet assembly is off, you'll want to sand off the mushroomed edges and sand the pole flat on top (I moved through a few grits of sandpaper -- I think 120 to do the work up to 400 to get nice and shiny).
Once that's done, it's theoretically as simple as adjusting the nut to change the height of the pole and get it level. I say theoretically because the nut is attached to the bolt with a metric s*8t-ton of loctite. I've read of people needing a blow torch and extra long handled wrench/big bench vice combo to get it to release. I used a torch and still never successfully got mine to release -- wrench and vice were not big enough, apparently. I lucked out by hitting about the right height when I sanded it in the first place.
Too much of a pain? The whole assembly can be had for 35 bucks new. The pole and bracket are two separate pieces.
If the head is mushroomed so much that it won't come down through the hole, you are in trouble. It's a bad idea to use a torch on the underside of your playfield (yeah, I know, obvious statement of the week), and it's hard to sand off the edges of the pole when the magnet is still in the playfield. I don't know the solution.
Once you get the magnet fixed, you are going to find that the hole bounce-outs are worse now that you are actually making direct shots to the hole. You'll want to put a piece of drop dead foam (think pinbits sells it) or weather insulation stripping both on the metal plate behind the hole and the wood platform behind the hole. All will be well. Then cover the magnet with molex (somebody sells a custom-sized piece, not on my usual computer to check now) so it doesn't mushroom again. Good luck.